An optical fiber is conventionally constituted of an optical core, which transmits an optical signal, and of an optical cladding, which confines the optical signal within the optical core. To that end the refractive index of the core, n0, is greater than the one of the cladding, nCl. An optical fiber is generally characterized by a refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference Dn between the refractive index at radius r, n(r), and the refractive index of the optical cladding nCl is shown on y-axis.
Nowadays, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode fiber, for a given wavelength, several optical modes can propagate simultaneously along the optical fiber, whereas in a single-mode fiber, the higher order modes (hereafter called HOMs) are cut-off or highly attenuated.
Single-mode fibers are commonly used for long-distance applications, such as access networks, metropolitan networks or long-haul networks. To obtain an optical fiber capable to transmit a single-mode optical signal, a core with a relatively small diameter is required (typically between 5 μm and 15 μm). To meet requirements of high speed or bit-rate applications (for example 10 Gbps), standard single-mode fibers require use of a modulated single-mode laser emitter tuned to work typically at a wavelength of 1550 nm. However, single-mode fibers suffer from nonlinearity problems, which are major limitations on fiber transmission capacity.
Multimode fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs) and multi-dwelling units (MDUs), more generally known as in-building networks. The core of a multimode fiber typically has a diameter of 50 μm, or 62.5 μm. The most prevalent multimode fibers in telecommunications are the refractive graded-index profile optical fibers. By minimizing the intermodal dispersion (i.e. the difference between the propagation delay times or group velocity of the optical modes along the optical fiber, also called DMGD for Differential Mode Group Delay), such a refractive index profile guaranties a high modal bandwidth for a given wavelength.
Since data traffic over fiber optic networks continues to grow exponentially, there is an increasing demand for increasing per-fiber traffic particularly across long distances. To this end, multiplexing techniques have been developed that allow a plurality of separate data streams to share the same optical fiber. Among these techniques, one promising approach is space division multiplexing (SDM), in which a plurality of data channels within a single optical fiber are provided by a respective plurality of optical signal modes guided by the fiber.
Such a technique has required the development of new types of optical fibers, called few-mode optical fibers, which support more than one spatial mode but fewer spatial modes than the multi-mode fibers. Such few-mode fibers, which are notably discussed in the PCT patent document WO2011/094400, support 2 LP (Linear Polarization) modes or more.
Space-division-multiplexed transmissions using Few-Mode Fibers (FMFs) have hence recently received considerable attention because of their potential to multiply the capacity of single-mode transmissions by the number of modes that will be used.
One approach to the design of Few-Mode Fibers consists of minimizing the Differential Mode Group Delays (DMGDs, i.e. the difference in the respective arrival times of the guided modes used for spatial multiplexing), so that all modes can be simultaneously detected using complex 2N×2N (N being the total number of spatial modes, i.e. including LP mode degeneracies) multiple-input-multiple-output (MIMO) techniques, regardless mode-coupling phenomena that is one of the limiting factor to bridge long distances.
In this approach, a careful design of the FMF is required, in order to reduce the DMGD (preferably below 300 ps/km to preserve MIMO efficiency) while still providing low bend losses for all guided LP modes.
This optimization, however, becomes more and more difficult when the number of LP modes increases.
So far, only FMFs supporting up to 20 usable LP modes with low Differential Modes Group Delays (DMGDs) have been reported.
In “50 μm Multimode Fibers for Mode Division Multiplexing” (proc. Ecoc 4.2.1-2015), P. Sillard et al. disclose 50 μm-diameter graded-index core multimode fibers, which can be adapted to mode-division-multiplexed transmissions that use MIMO digital signal processing and selective mode multiplexing. Such fibers were realized and characterized and compared to low-differential-mode-group-delay few-mode fibers.
FIG. 1 illustrates the refractive index difference with respect to the radius of such a FMF with a core diameter of 50 μm that supports 30 LP modes at 1550 nm but in which only 20 LP modes are usable. Actually, a severe degradation of the bend losses prevents the use of the 9th and 10th mode groups in space-division-multiplexed systems for such fibers.
Patent document US 2015/0168643 discloses a few-mode fiber, having a graded-index core and a surrounding cladding comprising a layer between the core and the trench, a down-doped trench abutting the layer, and an undoped cladding region abutting the trench. The fiber's refractive index profile is configured to support 9 to 20 LP modes for transmission of a spatially-multiplexed optical signal. Undesired modes have respective effective indices that are close to, or less than, the cladding index so as to result in leakage of the undesired modes into the outer cladding. The index spacing between the desired mode having the lowest effective index and the leaky mode with the highest effective index is sufficiently large so as to substantially prevent coupling therebetween.
Although such designs are promising, they do not allow supporting 25 or 30 usable LP modes while reducing the Differential Mode Group Delays as much as desired. In addition, the profiles disclosed in both documents are not optimized to ensure low bend losses, which, however, are mandatory for FMFs.
Accordingly, a need exists for designs for Few-Mode optical Fibers guiding an increased number of supported modes (25 LP modes or more), with small differential mode group delays between any combination of LP guided modes (preferably below 200 ps/km) and low bend losses (preferably below 100 dB/turn at 10 mm bend radius).